Currently, a cold cathode tube represents a principal vehicle for a light source of an illumination apparatus used in non-light emitting transparent display devices such as liquid crystal displays. Currently, from the points of view of wide color reproduction and environmental considerations (mercury less), light emitting diodes (hereinafter, referred to as LEDs) are desirable as light sources which can replace cold cathode tubes. Especially, when using liquid crystal displays, if the red color LED, green color LED, and blue color LED are used in the light source of the illumination apparatus, because color reproduction is flourishing, the application uses of LEDs have been actively investigated.
Because the emitting light intensity per LED is small compared to that of the cold cathode tube, in order to obtain a desired luminescence, it is necessary to arrange a plurality of LEDs within the illumination apparatus. When a plurality of LEDs is arranged within an illumination apparatus, with a uniform arrangement interval for every LED, the same junction temperature for all LEDs, and the same driving conditions (pulse height of the driving current and the pulse width of the driving current are equal), it is thought that the LEDs can be lit. However, with this arrangement, the luminescence of the light emitting surface of the illumination apparatus becomes dark near the ends compared to the center part. This darkness results from the superposition of light being reduced in the direction of the ends.
Consequently, in order to make the luminescence of the light emitting surface of the illumination apparatus uniform at the center part and at the ends, proposals have been made to narrow the arrangement interval of the LEDs, when approaching the ends from the center part, as disclosed in Patent Documents 1 and 2, below. According to the arrangement of these LEDs, the luminescence and chromaticity of the center part and ends of the light emitting surfaces of the illumination apparatus can be made uniform.
However, LEDs have characteristics for which the emitting light intensities, with respect to the junction temperature, as shown in FIG. 1, differ with every color. Here, the junction temperature is the temperature of the pn junction and is called the junction temperature.
Generally, the construction for the red color LED and the green and blue color LED differs. Consequently, as shown in FIG. 1, the reduction in the emitting light intensity of the red color LED (curve 1a) which follows an increase in junction temperature is large compared to the emitting light intensity (curve 1b) of the green and blue color light. Furthermore, with a reduction in the emitting light intensity for continuous lighting, the junction temperature assumes a high level and accelerates, as shown in FIG. 2.
That is, in FIG. 2, for example, a relationship exists between the emitting light intensity (curve 2a) of LEDs placed on the upper part of the illumination apparatus at which the junction temperature becomes 70° C., the emitting light intensity (curve 2b) of the LED placed at the center part of the illumination apparatus at which the junction temperature becomes 65° C., the emitting light intensity (curve 2c) placed at the lower part of the illumination apparatus at which the junction temperature becomes 60° C., and the lighting period. Here, the upper part is in the upward direction, the lower part is in the downward direction, and the center part is between the upward and downward directions of the illumination apparatus, when arranging the liquid crystal display in a nearly perpendicular direction.
Thus, the junction temperature of the LED is considered to be within the illumination apparatus. Normally, the liquid crystal display is placed perpendicular to the level surface. When placed with this perpendicular orientation, the specific gravity becomes lighter as air that has been warmed from heat generated by LEDs rises. From this rising of the air, the temperature (below, called the base material temperature) of the base materials on the upper part becomes high compared to that at the center part, and the lower part base material temperature becomes low compared to that at the center part. The LED junction temperatures under these conditions are positionally different, namely, as shown in FIG. 3, the junction temperature of the LED on the upper part (curve 3a) becomes higher than the junction temperature at the center part (curve 3b), and the junction temperature of the LED on the lower part (curve 3c) becomes lower than the junction temperature (curve 3b) of the center part. Differences in the LED junction temperatures are generated at the upper part, center part, and lower part, during the lighting period.
In these instances of position dependent temperatures, it is possible to obtain the LED junction temperatures using the following equation (1).LED junction temperature=Rth×Vf×If×D+Tb  (1)wherein Rth represents heat resistance between the LED junction and the base material, Vf; the voltage applied in LED order, If; pulse height of the LED drive current, D; pulse width of the LED drive current, and Tb; base material temperature.
Here, FIG. 4 is a cross-section explaining the luminescence and color irregularities caused by LED junction temperatures when the LEDs are arranged at uniform intervals within the display device. As shown in FIG. 4, directly after lighting starts (for example, time A in FIG. 3), the junction temperature of the LEDs at the outer periphery of the screen rises, and the luminescence at the outer periphery falls. As time passes (for example, after 100 minutes (time B in FIG. 3), after 30,000 hours (time C in FIG. 2)), the junction temperature of the upper part LEDs is higher than those at the center part, and the junction temperature of the lower part LEDs is lower than those at the center part. The reduction in the amount of luminescence of the red color LED is, therefore, greater than the reduced amount of luminescence experienced by the blue and green color LEDs on the upper part. Consequently, the reduction of the luminescence on the upper part becomes great and color irregularities appear. The reduction in the amount of luminescence accelerates from continuous lighting, the junction temperature, as shown in FIG. 2, is at a high level and the luminescence and color of the light emitting surface of the illumination apparatus changes, as shown in FIGS. 2 and 3. The junction temperature of the LEDs at the upper part is normally high compared to those LEDs at the center part, and with the passage of time, the junction temperature of upper part LEDs rises.
FIG. 5 is a cross-section explaining the luminescence and light irregularities caused by the junction temperature of the LEDs of the illumination apparatus in a construction which has narrowed the arrangement intervals of the LEDs as the outer periphery is approached from the center part within the illumination apparatus. As shown in FIG. 5, directly after lighting starts (for example, at time A in FIG. 3) the junction temperatures are nearly uniform. However, with the passage of time (for example, after 100 minutes (time B in FIG. 3), after 30,000 hours (time C in FIG. 2)), the junction temperature of the upper part LEDs is high compared to the junction temperatures of the center part LEDs, and the junction temperature of the lower part LEDs is lower than the junction temperatures of the center part LEDs. Furthermore, the reduction in the amount of luminescence accelerates from continuous lighting, the junction temperature, as shown in FIG. 2, is at a high level and the luminescence and color irregularities on the upper part of the light emitting surface of the illumination apparatus become more pronounced.
Thus, also with the passage of time, as a first method for making uniform the luminescence and color of the light emitting surface of the illumination apparatus, it is thought to make the pulse height of the LED drive currents higher at the upper part than at the center part, and lower at the lower part than at the center part. It is also thought to make the pulse width of the LED drive currents wider on the upper part than at the center part, and narrower on the lower part than at the center part, or to execute both the pulse height and pulse width strategies.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication, No. 2006-120644    [Patent Document 2] Japanese Unexamined Patent Application, First Publication, No. 2006-189665
However, using the previously described first method, the temperature differences between LEDs increases, because of the discrepancy in energy supplied to each LED, with the passage of time. Consequently, the reduction in emitting light intensity from continuous lighting is faster on the upper part than at the center part, and slower on the lower part than at the center. Because of these differences, normally, in order to maintain luminescence and color uniformity of the light emitting surface of the illumination apparatus, circuits adjusting the pulse height or pulse width or both of the LED drive currents, are required. Problems resulting from this requirement are reflected in an increase in the cost of materials and an increase in packaging space.
In addition, a second method can be considered where a heat dissipation means is provided to make LED junction temperatures uniform. However, even with this second method, accompanying cost and packaging space increase.